Add a feature allowing base files creation

[simonc]

Sometimes you need to have already existing directories and files for your tests.
Being able to provide a base listing of it would be useful.

Two formats could be possible:

/test-dir/other-dir/file.rb         # would create the file and parent directories
/test-dir/other-dir2/               # would create the directory and its parent
...

Or

/test-dir/
  other-dir/
    file.rb
  other-dir2/

The first one is simpler and probably easier to read in the end.